Example:
https://travis-ci.org/python/cpython/jobs/590339147

$ python --version

Python 3.6.3

$ pip --version

pip 9.0.1 from /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages (python 3.6)

Could not locate requirements.txt. Override the install: key in your .travis.yml to install dependencies.
before_script.1

0.00s$ cd Doc
before_script.2

8.58s$ python -m pip install sphinx~=1.6.1 blurb

1.88s$ make check suspicious html SPHINXOPTS="-q -W -j4"

python3 tools/rstlint.py -i tools -i ./venv -i README.rst

No problems found.

make[1]: Entering directory `/home/travis/build/python/cpython/Doc'

mkdir -p build

Building NEWS from Misc/NEWS.d with blurb

PATH=./venv/bin:$PATH sphinx-build -b suspicious -d build/doctrees -D latex_elements.papersize= -q -W -j4 . build/suspicious 

Sphinx version error:

This project needs at least Sphinx v1.8 and therefore cannot be built with this version.

make[1]: *** [build] Error 1

make[1]: Leaving directory `/home/travis/build/python/cpython/Doc'

Suspicious check complete; look for any errors in the above output or in build/suspicious/suspicious.csv.  If all issues are false positives, append that file to tools/susp-ignored.csv.

make: *** [suspicious] Error 1

The command "make check suspicious html SPHINXOPTS="-q -W -j4"" exited with 2.
cache.2

store build cache